Involvement of extracellularly bound calcium in the activation of arterial smooth muscle.

The role of extracellularly bound Ca in the activation of rabbit aorta by the PGH2 analogue U-44069 was assessed using 45Ca flux determinations. The 45Ca influx process, which is activated by U-44069, was found to exhibit two phases: an initial rapid phase which was transient and a slower phase which was sustained. The 45Ca-40Ca exchange labeling rate of the Ca pool which enters the cell during the initial phase of Ca influx was found to be much slower than that of the free extracellular Ca2+, but quite similar to that of an extracellularly bound Ca pool. These findings suggest that an extracellularly bound Ca pool, possibly located on the outer surface of the plasmalemma, is involved in the U-44069-stimulated Ca influx process.